Attention is being drawn to nanocellulose, a new material that has a fiber diameter as small as less than 1 μm achieved by disintegrating a fiber constituting a plant-derived cellulose material. Among such materials as called nanocellulose, several materials in different sizes or aspect ratios have been proposed. In particular, the materials having a fiber diameter of about 4 to 100 nm and a length of 5 μm or more, often called cellulose nanofiber, cellulose nanofibrils, or fibrillated cellulose, have been the target of ongoing manufacture and research due to their excellent properties as reinforcing fibers. While various names are proposed for such materials, they are called cellulose nanofiber herein. In addition, materials belonging to modified cellulose and having a size and aspect ratio similar to those of cellulose nanofiber are collectively called nanofibers herein.
To produce cellulose nanofiber, cellulose has to be finely defibrated. Not only because cellulose itself is a strong material but also because wood-derived cellulose contains lignin and other strong substances, it is difficult to defibrate cellulose into cellulose nanofiber of an appropriate size. To overcome this problem, various methods have been proposed.
For example, Non-Patent Document 1 listed below reports a defibrating method with a high-pressure homogenizer or the like, the method including pressing a pulp slurry into a small void space and releasing pressure to allow the defibration to occur. However, such mechanical method is problematic in that defibration requires a great deal of energy because treatment has to be repeated many times to cause the defibration to proceed. Such method is also problematic in that capable machines are limited and the obtained cellulose nanofiber is susceptible to damage because it is difficult to obtain fibers having a diameter of the order of several nanometer and treatment with a high pressure of 200 MPa or higher is repeated.
Patent Document 1 listed below proposes a defibrating method that includes pretreatment of raw materials with acid to remove lignin so that the subsequent defibration is facilitated. This method can reduce an applied external force and thus less damaged cellulose nanofiber is obtained, compared with a simple mechanical method that includes applying high pressure.
Non-Patent Document 2 and Patent Document 2 listed below each report a technique for facilitating defibration by introducing a carboxyl or carboxymethyl group into cellulose. When pulp is treated in the presence of a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and sodium hypochlorite as an oxidant, a carboxyl group can be efficiently introduced to C-6 position of the cellulose constituting fibers on the surface of cellulose microfibrils (Non-Patent Document 2). A carboxymethyl group can also be introduced by reacting pulp with chloroacetic acid sodium salt in the presence of alkali (Patent Document 2). Both TEMPO-oxidized cellulose containing an introduced carboxyl group and carboxymethyl cellulose containing an introduced carboxymethyl group can be defibrated more easily than unmodified cellulose. Consequently, a product equivalent to a nanofiber in size can be obtained with fewer treatments at lower pressure than a simple mechanical method involving high-pressure treatment.